The invention relates to a process for taking out of operation a catalyst-containing reactor for the catalytic hydrogenation of a fluid i.e. a liquid or gaseous feed, in which hydrogenation the feed is passed over a catalyst in the presence of added hydrogen. In general, hydrogenations of this type will be carried out at a temperature above 300.degree. C. In many cases the feed will contain sulphur compounds, from which H.sub.2 S is formed during hydrogenation.
The above-mentioned catalytic hydrogenations are often applied in the refining of mineral oil, but are also applied outside that field.
An example of the use of fluid feed is the catalytic desulphurization of certain petroleum fractions, such as a distillate fraction or a residual fraction. In this process the feed together with hydrogen is passed through a reactor filled with desulphurization catalyst at a temperature at which the feed is not yet or only slightly converted by cracking or isomerization. Sulphur compounds present in the liquid feed are hydrogenated with the formation of hydrogen sulphide. A desulphurized feed on the one hand and a hydrogen sulphide and unused hydrogen-containing gas on the other are discharged from the reactor. The latter gas, for example, can be passed to a Claus plant described below.
An example of catalytic hydrogenation involving a gaseous feed is the catalytic reduction of a Claus off-gas originating from a Claus plant for the preparation of elemental sulphur by reaction of sulphur dioxide and hydrogen sulphide. Claus off-gas still contains a percentage of unreacted sulphur compounds which must usually be removed, which may be effected by total reduction of the sulphur compounds with hydrogen to hydrogen sulphide, after which the hydrogen sulphide formed is removed from the Claus off-gas (for example by absorption) and is recycled to the Claus plant. Claus plants are found not only in refineries but also in natural gas fields, for processing the hydrogen sulphide removed from the natural gas.
A phenomenon which almost invariably occurs during the catalytic hydrogenation of sulphur compounds is the deposition of carbon on the catalyst. This carbon often originates either from hydrocarbons of the feed (for example in the desulphurization of petroleum fractions) or from combustion gases admixed for heating (for example at the reduction of Claus off-gas). In the desulphurization of liquid products, for example, tarry products are also deposited on the catalyst.
Solid particles, such as scale, silica, metal salts and iron particles, are usually deposited at the beginning of the catalyst bed, the sulphides and carbon being present over the entire length in the catalyst bed.
In the regeneration of this catalyst the carbon and the tarry products are burnt off, while sufficient oxygen mixed with a large quantity of steam or nitrogen is invariably supplied to maintain the temperature in the reactor at an acceptable level. Temperatures applied in practice often lie between 300.degree. C. and 500.degree. C., since the carbon is not burnt off below this temperature. In this regeneration the sulphides are oxidized simultaneously with the carbon and the tar and with a view to the desirability of limited heat generation this oxidation may only proceed very gradually. The regeneration, therefore, usually takes a very long time, with large reactors up to some days.
In some cases, for example in the desulphurization of residual petroleum fractions, the total quantity of catalyst in the reactor is very large, for example 500 tons. In these cases the quantity of catalyst on which no solid particles have been deposited becomes, in an absolute sense, very large compared with the quantity of catalyst on which such deposition has occurred. A drawback is then that of the total quantity of catalyst the metal sulphides present are oxidized in order to cause the carbon and the tarry products to be burnt off.
When the reactor must be opened for some reason or other, it will not be sufficient for the reactor to be cooled and purged with an inert gas, as would be expected. It has, in fact, been found that when the reactor is purged with low-temperature inert gas for a long period of time and subsequently opened, the catalyst begins to glow in the open air and releases sulphur dioxide in case sulphur compounds have been present in the feed. Under these conditions the catalyst content consequently has a pyrophoric character, and the performance of work on or in the reactor is impossible or dangerous.
Up to now this problem has been solved by fully regenerating the catalyst content before opening the reactor. This solution, however, is expensive and time-consuming and the invention aims at providing other routes.
For example, it has been found to occur in practice that the reactor must be opened, while the activity of the catalyst has not yet decreased to such an extent that the catalyst requires regenerating. Another object of the invention is to provide a process in which this regeneration is not necessary in such cases and the reactor can nevertheless be opened.
It has now been found that the pyrophoric character of the catalyst is entirely removed by performing a fully controlled oxidation, in which measures are taken to maintain the temperature continuously below 300.degree. C.
Surprisingly, this "passivation" of the catalyst takes place without the active sulphides, such as cobalt, nickel, molybdenum and/or tungsten sulphide, being oxidized. The passivation, therefore, takes considerably less time than the regeneration customary hitherto, in which latter treatment these sulphides are in fact oxidized.
It has further been found that also the carbon and the tarry products are hardly if at all oxidized in the passivation. There are distinct indications that the catalyst derives its pyrophoric character from the presence of finely divided iron and/or iron sulphide, which seems to be oxidized at low temperature.